An optical disc, such as a compact disc (CD), is an electronic data storage medium that can be written to and read using a low-powered laser beam. Optical disc technology first appeared in the marketplace with the CD, which is typically used for electronically recording, storing, and playing back audio, video, text, and other information in digital form. A digital versatile disc (DVD) is another more recent type of optical disc that is generally used for storing and playing back movies because of its ability to store much more data in the same space as a CD.
Compact Discs were initially a read-only storage medium that stored digital data as a pattern of bumps and flat areas impressed into a piece of clear polycarbonate plastic through a complex manufacturing process. However, average consumers can now burn their own CDs with CD players capable of burning digital data into CD-Rs (CD-recordable discs) and CD-RWs (CD-rewritable discs).
Methods for labeling the non-data side of such optical discs with text and images, for example, have continued to develop as consumers desire more convenient ways to identify their own recorded discs. Basic methods for labeling a disc include physically writing on the non-data side with a permanent marker (e.g., a Sharpie marker) or printing out a paper sticker label and sticking it onto the non-data side of the disc. Other physical marking methods developed for implementation in conventional CD players include ink jet, thermal wax transfer, and thermal dye transfer methods. Still other methods use the laser in a conventional CD player to mark a specially prepared CD surface. Such methods apply equally to labeling CDs and DVDs.
A label image can be rendered on the label surface (i.e., the non-data side, or top side) of an optical disc by marking the label surface with a laser beam along concentric circular tracks around the disc. For each track, spots of constant size and optical density are marked by the laser according to the marking data for that track. The rotational speed of the spindle is adjusted to correspond with the time it takes for the media (i.e., the coating on the disc) to respond (i.e., change color) to the energy from the laser. However, the media response time can require a spindle speed slow enough that it puts a constraint on the rotational speed accuracy of the spindle motor.
A CD drive typically employs an inexpensive 3-phase motor with an integer position encoder as its motor drive method. Three built-in Hall effect encoders provide speed sensing through 18 feedback positions that exploit the rising and falling edges of the Hall sensor signals. At typical rotational speeds, on the order of 1000 RPM, speed accuracy can be adequately maintained with this degree of feedback. However, at slower rotational speeds such as speeds that are conducive to marking the label surface of a disc, the number of feedback positions from the Hall sensors is too few to maintain speed accuracy. Therefore, a more reliable and repeatable speed control is needed.